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Disulfide bond formation on luminal proteins in thylakoids 1240 V. Conclusion 1242 Acknowledgements 1242 References 1242 SUMMARY: Disulfide bonds are post-translational modifications crucial for the structure and function of thousands of proteins. Their formation and isomerization, referred to as oxidative folding, require specific protein machineries found in oxidizing subcellular compartments, namely the endoplasmic reticulum and the associated endomembrane system, the intermembrane space of mitochondria and the thylakoid lumen of chloroplasts. At least one protein component is required for transferring electrons from substrate proteins to an acceptor that is usually molecular oxygen. For oxidation reactions, incoming reduced substrates are oxidized by thiol-oxidoreductase proteins (or domains in case of chimeric proteins), which are usually themselves oxidized by a single thiol oxidase, the enzyme generating disulfide bonds de novo. By contrast, the description of the molecular actors and pathways involved in proofreading and isomerization of misfolded proteins, which require a tightly controlled redox balance, lags behind. Herein we provide a general overview of the knowledge acquired on the systems responsible for oxidative protein folding in photosynthetic organisms, highlighting their particularities compared to other eukaryotes. Current research challenges are discussed including the importance and specificity of these oxidation systems in the context of the existence of reducing systems in the same compartments.

International audience; Land Surface Phenology (LSP) and Leaf Area Index (LAI) are important variables that describe the photosynthetically active phase and capacity of vegetation. Both are derived on the global scale from optical satellite sensors and require robust validation based on in situ sensors at high temporal resolution. This study assesses the PAI Autonomous System from Transmittance Sensors at 57 degrees (PASTiS-57) instrument as a low-cost transmittance sensor for simultaneous monitoring of LSP and LAI in forest ecosystems. In a field experiment, spring leaf flush and autumn senescence in a Dutch beech forest were observed with PASTiS-57 and illumination independent, multi-temporal Terrestrial Laser Scanning (TLS) measurements in five plots. Both time series agreed to less than a day in Start Of Season (SOS) and End Of Season (EOS). LAI magnitude was strongly correlated with a Pearson correlation coefficient of 0.98. PASTiS-57 summer and winter LAI were on average 0.41 m(2)m(-2) and 1.43 m(2)m(-2) lower than TLS. This can be explained by previously reported overestimation of TLS. Additionally, PASTiS-57 was implemented in the Discrete Anisotropic Radiative Transfer (DART) Radiative Transfer Model (RTM) model for sensitivity analysis. This confirmed the robustness of the retrieval with respect to non-structural canopy properties and illumination conditions. Generally, PASTiS-57 fulfilled the CEOS LPV requirement of 20% accuracy in LAI for a wide range of biochemical and illumination conditions for turbid medium canopies. However, canopy non-randomness in discrete tree models led to strong biases. Overall, PASTiS-57 demonstrated the potential of autonomous devices for monitoring of phenology and LAI at daily temporal resolution as required for validation of satellite products that can be derived from ESA Copernicus' optical missions, Sentinel-2 and -3.

Croissance et capacités photosynthétiques de peupliers et d’aulnes dans une plantation agroforestière composée de mélanges d’espèces fixatrices / non fixatrices d’azote

The aim of this study was to systematically analyze the potential and limitations of using plant functional trait observations from global databases versus in situ data to improve our understanding of vegetation impacts on ecosystem functional properties (EFPs). Using ecosystem photosynthetic capacity as an example, we first provide an objective approach to derive robust EFP estimates from gross primary productivity (GPP) obtained from eddy covariance flux measurements. Second, we investigate the impact of synchronizing EFPs and plant functional traits in time and space to evaluate their relationships, and the extent to which we can benefit from global plant trait databases to explain the variability of ecosystem photosynthetic capacity. Finally, we identify a set of plant functional traits controlling ecosystem photosynthetic capacity at selected sites. Suitable estimates of the ecosystem photosynthetic capacity can be derived from light response curve of GPP responding to radiation (photosynthetically active radiation or absorbed photosynthetically active radiation). Although the effect of climate is minimized in these calculations, the estimates indicate substantial interannual variation of the photosynthetic capacity, even after removing site-years with confounding factors like disturbance such as fire events. The relationships between foliar nitrogen concentration and ecosystem photosynthetic capacity are tighter when both of the measurements are synchronized in space and time. When using multiple plant traits simultaneously as predictors for ecosystem photosynthetic capacity variation, the combination of leaf carbon to nitrogen ratio with leaf phosphorus content explains the variance of ecosystem photosynthetic capacity best (adjusted R-2 = 0.55). Overall, this study provides an objective approach to identify links between leaf level traits and canopy level processes and highlights the relevance of the dynamic nature of ecosystems. Synchronizing measurements of eddy covariance fluxes and plant traits in time and space is shown to be highly relevant to better understand the importance of intra-and interspecific trait variation on ecosystem functioning.

Plants with lowered activity of enzymes catalyzing the final step of ascorbate synthesis or the equilibrium between reduced and oxidized forms have highly pleiotropic phenotypes suggesting regulatory roles for ascorbate and its oxidized forms. Network analysis of the transcriptome, proteome and key metabolites of RNAi lines for ascorbate oxidase, monodehydroascorbate reductase and galactonolactone dehydrogenase has been carried out in orange fruit pericarp of tomato (Solanum lycopersicum). We show a transcriptional switch-type response with expression profiles in ascorbate oxidase lines being inversed compared to the monodehydroascorbate reductase and galactonolactone dehydrogenase lines. Differentially expressed genes are linked to ribosome biogenesis and translation. The inversion between the transcriptome signatures is not specific to tomato fruit as it is conserved in Arabidopsis. The transcriptome response is not correlated with accumulated proteins which, with the metabolites, are correlated to the activity of the ascorbate redox enzymes. Differentially accumulated proteins include metacaspase, protein disulphide isomerase, chaperone DnaK and carbonic anhydrase and the metabolites chlorogenic acid, dehydroascorbate and alanine. Hub genes are related to signaling, the heat-shock response and ribosome biogenesis. This study in a non-photosynthetic tissue reveals signaling from the ascorbate pool related to the heat-shock response, translation and protein synthesis.

International audience; Drought frequency and intensity are expected to increase in the Mediterranean as a consequence of global climate change. To understand how photosynthetic capacity responds to long-term water stress, we measured seasonal patterns of stomatal (S-L), mesophyll (MCL) and biochemical limitations (B-L) to net photosynthesis (A(max)) in three Quercus ilex (L.) populations from sites differing in annual rainfall. In the absence of water stress, stomatal conductance (g(s)), maximum carboxylation capacity (V-cmax), photosynthetic electron transport rate (J(max)) and A(max) were similar among populations. However, as leaf predawn water potential (Psi(l,pd)) declined, the population from the wettest site showed steeper declines in g(s), V-cmax, J(max) and A(max) than those from the drier sites. Consequently, S-L, MCL and B-L increased most steeply in response to decreasing Psi(l,pd) in the population from the wettest site. The higher sensitivity of A(max) to drought was primarily the result of stronger stomatal regulation of water loss. Among-population differences were not observed when g(s) was used instead of Psi(l,pd) as a drought stress indicator. Given that higher growth rates, stature and leaf area index were observed at the wettest site, we speculate that hydraulic architecture may explain the greater drought sensitivity of this population. Collectively, these results highlight the importance of considering among-population differences in photosynthetic responses to seasonal drought in large scale process-based models of forest ecosystem function

International audience; Because all microclimatic variables change with elevation, it is difficult to compare plant performance and especially photosynthetic capacity at different elevations. Indeed, most previous studies investigated photosynthetic capacity of low- and high-elevation plants using constant temperature, humidity and light but varying CO2 partial pressures (PCO2). * Using gas exchange measurements, we compared here maximum assimilation rates (Amax) at ambient and constant-low-elevation PCO2 for two temperate tree species along an altitudinal gradient (100 to 1600 m) in the Pyrénées mountains.* Significant differences in Amax were observed between the CO2 partial pressure treatments for elevations above 600 m, the between-treatment differences increasing with elevation up to 4 μmol m−2 s−1. We found an increase in Amax with increasing elevation at constant-low-elevation PCO2 but not at ambient PCO2 for both species. Given a 10% change in PCO2, a proportionally higher shift in maximum assimilation rate was found for both species. * Our results showed that high elevation populations had higher photosynthetic capacity and therefore demonstrated that trees coped with extreme environmental conditions by a combination of adaptation (genetic evolution) and of acclimation. Our study also highlighted the importance of using constant CO2 partial pressure to assess plant adaptation at different elevations.; Les conditions microclimatiques étant très variables avec l'altitude, il est difficile de comparer les performances d'une espèce végétale à différentes altitudes, particulièrement la capacité photosynthétique. En effet, la plupart des études antérieures ont estimé le taux maximal d'assimilation à basses et hautes altitudes en maintenant la température, l'humidité de l'air et la lumière constantes mais en laissant varier la pression partielle de CO2 (PCO2). * Afin de comparer le taux maximum d'assimilation (Amax) à pressions partielles de CO2 constantes de basse altitude et variables, nous avons effectué des mesures d'échanges gazeux sur deux espèces d'arbres tempérés le long d'un gradient altitudinal de 1600 m de dénivelé dans les Pyrénées françaises. * La différence entre les deux traitements de PCO2 est significative au-dessus de 600 m d'altitude et atteint un maximum de 4 μmol m−2 s−1. Pour les deux espèces, nous avons mis en évidence une augmentation de Amax avec l'altitude à PCO2 constantes mais pas à PCO2 ambiantes. Pour une modification de PCO2 de 10 %, le changement du taux maximum d'assimilation est proportionnellement supérieur chez les deux espèces. * Nos résultats montrent que les populations de hautes altitudes possèdent une capacité photosynthétique supérieure, démontrant que les arbres font face aux conditions environnementales extrêmes grâce à des adaptations génétiques ou des acclimatations. Notre étude souligne ainsi l'importance de fixer la PCO2 pour comparer l'adaptation des plantes à différentes altitudes.

Les objectifs de ce travail de thèse étaient : (i) identifier les principaux compromis et covariations d’un ensemble de traits foliaires relatifs à l’acquisition du carbone et (ii) caractériser la variabilité de ces traits en réponse à l’éclairement (plasticité phénotypique) et la diversité interspécifique de cette plasticité chez des espèces de forêt tropicale humide. Les mesures ont été réalisées sur des semis de 13 espèces d’arbres élevés en conditions contrôlées dans 3 traitements d’éclairements relatifs distincts. Une analyse coût-bénéfice des traits du bilan de carbone foliaire a été centrée sur les coûts de construction du métamère (CCmetm), le temps de retour sur investissement (PBT) et la mesure des durées de vie des feuilles (LLS). Une forte diversité interspécifique a été observée pour l’ensemble des traits étudiés. Les espèces se répartissent le long d’un axe opposant durée de vie des feuilles et LMA (masse surfacique) d’un côté, aux capacités photosynthétiques (Asat), PNUE (Asat/N), respiration (Rd) et teneurs en azote (Nm) de l’autre. Ces résultats sont en adéquation avec le schéma universel d’acquisition des ressources proposé par Wright et al. (2004). Les relations entre LLS et PBT sont faibles et l’ensemble des espèces amortissent très largement leur CCmetm. De manière générale, le classement des espèces est maintenu d’un traitement à l’autre et, à l’exception des coûts de construction, seuls de faibles effets d’interaction ont été observés entre espèces et traitements. Le calcul d’un indice de plasticité a également permis de montrer qu’en dépit d’une certaine diversité interspécifique de la plasticité phénotypique, aucune espèce n’était globalement plus « plastique » que les autres sur l’ensemble des traits considérés. Enfin, si les espèces pionnières se distinguent nettement par leurs attributs, leur degré de plasticité n’est pas différent de celui des espèces de sous-bois., The aim of this research were (i) identify the main trade-off and covariations of a group of leaf traits related to carbon assimilation, and (ii) assess the irradiance-elicited plasticity, and test whether this plasticity differs among species of tropical rain forest. Measurements were made with saplings from 13 species growing under three levels of irradiance, provided by shading nets, in an open green-house. A cost benefit analysis was undertaken to quantify carbon balance of leaves metamers. Construction costs of metamers (CCmetm), payback time of the investment of carbon into leaf and metamer structure (PBT), and leaf life span (LLS) were recorded on the saplings. A large inter-specific diversity was recorded for all the traits, the species are scattered along a principal component axis opposing on one hand LSS and leaf mass-to-area ratio (LMA), and on the other hand, photosynthetic capacity, photosynthetic nitrogen use efficiency (PNUE), respiration (Rd) and leaf nitrogen content (N). These results are in agreement with the concept of “leaf economics spectrum” suggested by Wright et al. (2004). The relationship between LLS and PBT were very loose, which shows that all species fully cover the construction costs, even under very low irradiance. Species ranking was not modified by irradiance whatever the trait, and, except for specific construction costs, only very weak interactions were found between species and irradiance. The computation of a plasticity index allowed to demonstrate that despite some inter-specific diversity of the values taken by the index for some traits, there was no consistent difference among species, and none of them appeared consistently more plastic than the others. Even if the two pioneer species analysed here differed largely from the others, from the point of view of trait values, they displayed similar levels of plasticity.

Links between architectural stages of development (ASD), photosynthetic capacities and leaf characteristics in Tachigali melinonii (Caesalpiniaceae) were investigated in the tropical forest of French Guiana. Three ASD were sampled under three light conditions to elucidate the impact of ASD- and light-related changes on photosynthetic functional traits. Photosynthetic parameters (maximum rate in RubisCO carboxylation (Vcmax) and maximum rate of electron transport (Jmax) were estimated by adjusting a biochemical model (Farquhar et al., 1980) to response curves of CO2 assimilation rate to intercellular CO2 concentration, under saturated light. In addition, N and C foliar concentrations were analysed and N allocations to the carboxylation and bioenergetic processus of photosynthesis were calculated using the model of Niinemets and Tenhunen (1997). The variations in ecophysiological parameters observed were due to leaf anatomical and structural changes instead of changes in photosynthetic components, suggesting adjustments to light conditions. Tachigali melinonii thus appears to display morphological and anatomical plasticity at different ASDs and under different light conditions., Une espèce forestière de Guyane Française, Tachigali melinonii (Caesalipiniaceae), a été choisie afin de regarder la variation des traits fonctionnels relatifs à la photosynthèse en fonction de la disponibilité en lumière et en fonction de son stade de développement architectural. Du fait du tempérament de l'espèce vis-à-vis de la lumière de type «struggling gambler» (ombrage nécessaire aux jeunes stades puis développement en pleine lumière), trois stades clefs de développement parmi la séquence de différenciation de l'espèce ainsi que trois environnements lumineux ont été choisis. Les capacités photosynthétiques foliaires ont été estimées à partir de l'ajustement de paramètres du modèle biochimique de photosynthèse foliaire (Farquhar et al. 1980) sur des courbes de réponses de l'assimilation de CO² (A) à des concentrations intracellulaires en CO² (Ci) croissantes, en condition de lumière saturante. La vitesse maximale de carboxylation Vcmax et le flux maximal d'électrons photosynthétiques Jmax ont ainsi été estimés et ensuite reliés aux caractéristiques structurales foliaires, notamment la concentration en azote, en carbone ainsi qu'avec la densité et l’épaisseur. Les fractions d'azote allouées aux processus photosynthétiques de carboxylation et de bioénergétique ont été calculées. Des variations, dues à un effet du stade de développement sur les paramètres étudiés ainsi qu’à un effet de la quantité de lumière disponible, ont été constatées. Ces variations sont surtout liées à des ajustements anatomiques et structuraux foliaires et non à des modifications de l’appareil photosynthétique. Tachigali melinonii semble donc modifier ses caractéristiques structurales foliaires en fonction de son stade de développement, ce qui suppose un contrôle ontogénique partiel, et en fonction de la disponibilité en lumière.

L’évolution au cours de l’hiver de l’activité photosynthétique de rameaux de douglas, sapin pectiné et d’épicéa provenant de plantations est étudiée. La relation entre la photosynthèse nette et la température actuelle est similaire pour les 3 espèces avec un large domaine optimal de 5 à 25 °C. Les points inférieurs de compensation thermique de la photosynthèse sont de - 3 °C pour le douglas et - 5 °C pour les autres espèces. Pour les 3 espèces, on note un seuil thermique de dépression rémanente de la photosynthèse voisin de - 1, -2 °C. Les variations au cours de l’hiver de la capacité photosynthétique sont entièrement expliquées par celles des résistances internes à la diffusion du CO2. Le douglas et le sapin pectiné, espèces originaires de zones à hivers relativement doux et dont la résistance au gel est limitée, présentent une photosynthèse hivernale uniquement affectée, et de façon parfaitement réversible, par les variations accidentelles liées aux stress thermiques. La provenance d’épicéa étudiée, d’origine continentale et probablement extrêmement résistante au froid, révèle un modèle d’évolution dans lequel une tendance saisonnière de dépression hivernale de la photosynthèse se superpose aux variations accidentelles. Une discussion est engagée tendant à montrer que chacun de ces deux types de comportements pourrait être généralisé en fonction du type de résistance au gel., The winter-course of photosynthesis was studied on shoots of douglas fir, silver fir and Norway spruce cut from outside stands. The pattern of photosynthesic response to temperature was sinular for the 3 species with a wide optimal range from 5 to 25 °C. Low temperature limits were however - 3°C for douglas and - 5 °C for fir and spruce. Threshold values for tasting depressive effects of low temperatures were of about - 1, - 2 °C, irrespective of species. Douglas fir and silver fir, partially frost-resistant species and originating from mild winter climatic areas, exhibited a winter-course of photosynthesis affected only by changes in the thermal history of the shoots. In contrast, for the Norway spruce, for which a continental deep frost-resistant provenance was used, a more complex evolution pattern appeared, superposing a seasonal trend (winter depression) on the short term variations due to the thermal history. Discussions on the relevance of an extrapolation of the above-mentionned patterns with respect to the frost resistance characteristics are made.